The invention relates to an integrable circuit for frequency conditioning in a radio transceiver, in particular a cordless telephone, operating in an UHF range in particular at 900 MHz, and functioning as a time division duplex system. The time division duplex system alternately forms transmission and reception time slots with a frequency hopping technique that allocates a frequency to each time slot according to a defined scheme. The circuit is formed of amplifiers, filters and converters provided in a transmission path and a reception path. The transmission and reception paths are both alternately connected to a common antenna via a time division duplex switch that can be change over in a controlled manner.
Defined UHF frequency ranges can be used for free radio services in many states if specific, precisely prescribed conditions are adhered to. By way of example, in the USA the frequency range between 902 MHz and 928 MHz is available as the so-called industrial scientific and medical (ISM) band.
Radio services, that is to say e.g. cordless telephones, are permitted to be realized in the UHF-frequency band. In this case, however, it is necessary to adhere to the boundary conditions specified by the Federal Communications Commission (FCC) authority with respect to the channel spacing, the 20 dB bandwidth, the output power, etc.
There are essentially three system approaches for realizing a cordless telephone. The first of these approaches consists in an embodiment as a so-called direct sequence spread spectrum system. In this case, the radio signal is modulated with a high-bit-rate sequence of digital signals, the so-called pseudonoise (PN) code, in such a way that a broadband transmission signal results from the superposition. Each bit of the message signal to be transmitted is thus coded with the PN code in a transmitter, which PN code can be decoded again in a receiver that knows the PN code.
The second of the system approaches consists in the realization of a so-called low power system. In this case, there are no restrictive regulations below a transmission power of approximately 0 dBm.
The third system approach consists in an embodiment as a so-called frequency hopping spread spectrum system. In the case of the aforementioned ISM band, for example, these systems are permitted to have a maximum channel spacing of 500 kHz, where 99% of the power has to be emitted within this bandwidth. Consequently, at least 50 channels are available in that case, which are utilized on the basis of a pseudorandom sequence.
It is accordingly an object of the invention to provide an integrable circuit for the frequency conditioning of a radio transceiver, in particular a cordless telephone, operating in the UHF range which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a frequency conditioning architecture is specified which enables the highest possible integration level for a frequency hopping method. In particular, the intention is to specify an efficient frequency plan which avoids undesirable mixed products and at the same time enables an optimum time division multiplex (TDMA; Time Division Multiple Access) frame format.
Furthermore, the object of the invention is to realize the smallest possible channel spacing of 200 kHz to 300 kHz in order that dynamic channels on which severe interference occurs can be masked out. At the same time, the intention is not to exceed a frame period of 4 ms in order that echo compensation can be dispensed with.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for frequency conditioning in a radio transceiver operating in an ultra high frequency (UHF) range and functioning as a time division duplex system (TDD), the time division duplex system alternately forming transmission and reception time slots and has a frequency hopping technique (FH) allocating a frequency to each time slot according to a defined scheme, the circuit including:
an IF frequency synthesizer having a first phase-locked loop and a first phase-regulated HF oscillator for generating a directly modulated output signal;
a UHF frequency synthesizer having a second phase-locked loop and a second phase-regulated HF oscillator for generating an output frequency which is a UHF heterodyne frequency;
a first 1:n frequency divider receiving the output frequency of the UHF frequency synthesizer and dividing down the output frequency, the first 1:n frequency divider outputting a first divided UHF output frequency having a value of 1/n times that of the output frequency;
a single up-converter receiving the directly modulated output signal of the IF frequency synthesizer as an input frequency and the first divided UHF output frequency from the first 1:n frequency divider, the single up-converter generating an up-converted output signal;
a filter receiving the up-converter output signal and outputting a filtered up-converted output signal;
a transmission amplifier stage receiving the filtered up-converted output signal from the filter and outputs an amplified output signal, the IF frequency synthesizer, the UHF frequency synthesizer, the single up-converter, the filter and the transmission amplifier stage define a transmission path;
a time division duplex switch having a reception access path and a transmission access path receiving the amplified output signal from the transmission amplifier stage, the time division duplex switch is a controlled time division duplex switch that can be switch in a controlled manner;
a low-noise amplifier connected to the reception access path of the time division duplex switch;
a UHF reception filter connected to the low-noise amplifier;
a second 1:n frequency divider receiving the UHF heterodyne frequency from the UHF frequency synthesizer and generates a second divided UHF output frequency;
a first down-converter having a first input connected to the UHF reception filter and a second input receives the second divided UHF output frequency, the first down-converter outputs a first down-converted output signal;
a first IF bandpass filter of an upper IF plane receiving the first down-converted output signal and outputs a filtered down-converted output signal;
a second down-converter has an output, a first input receiving the filtered down-converted output signal and a second input receiving the directly modulated output signal generated by the IF frequency synthesizer and being present as a heterodyne frequency;
a second IF bandpass filter of a lower IF plane is connected to the output of the second down-converter;
a first amplifier connected to the second IF bandpass filter;
a third IF bandpass filter connected to the first amplifier;
a second amplifier connected to the third IF bandpass filter;
a demodulator connected to the second amplifier;
the IF frequency synthesizer, the UHF frequency synthesizer, the low-noise amplifier, the UHF reception filter, the first down-converter, the first IF bandpass filter, the second down-converter, the second IF bandpass filter, the first amplifier, the third IF bandpass filter, the second amplifier and the demodulator define a reception path; and
a common antenna connected to the reception path and the transmission path via the time division duplex switch.
In the inventive circuit, merely two frequency synthesizers are necessary in the circuit according to the invention. Consequently, a third frequency synthesizer is omitted, which considerably promotes a higher integration level.
Furthermore, the configuration of the circuit according to the invention allows the use of inexpensive external filters.
By virtue of the synthesis of the required UHF oscillator signal at at least twice the frequency as used in prior art circuits, the phase detector reference frequency is increased, with the result that the transient recovery time of the UHF frequency synthesizer can be halved.
Consequently, two transient processes can be accommodated within the same frame period. These facts make it possible to utilize the UHF frequency synthesizer at different frequencies for the transmission and reception slots, that is to say the frequency offset due to the second intermediate frequency is compensated for by the UHF frequency synthesizer.
It is expedient for the up-conversion that takes place in the transmission path to be effected by an up-converter with suppression of image frequencies, in which case the 90xc2x0 phase shift between the input signals can be generated by frequency dividers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an integrable circuit for the frequency conditioning of a radio transceiver, in particular a cordless telephone, operating in the UHF range, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.